1. Field of the Invention
The invention relates to a voltage-limiting circuit, and more particularly to a voltage-limiting circuit with energy recovery function for automatically limiting the amplitude of voltage at two ends of each power switching device in series connection.
2. Description of the Related Art
The electricity grid provides electric energy generally in the form of industrial-frequency (50 Hz or 60 Hz) AC energy for users, however, the electric energy practically used by users is different, such as in the form of DC (for example, electrolytic aluminum), high-frequency AC (for example, induction heating), and frequency voltage adjusted as required (for example, variable frequency driver for a motor). Thus a converter is required to convert electrical energy. Power switching devices is core components of the converter widely-used for realizing the conversion of electrical energy.
Due to the limitation of current level of the semiconductor industry, for example, the insulated gate bipolar transistor IGBT provided in batches at present has the withstand voltage limit value of 1700 V or 3300 V, and the high voltage such as 10 kV and 500 kV cannot be directly processed with a single power switching device. Furthermore, a low-voltage power switching device has lower cost, therefore, the low-voltage power switching devices are usually connected in series during the practical application to solve the problem of poor voltage resistance. The parameters of a power switching device and a control circuit thereof have discreteness including temperature sensitivity and time variance, and the length of time in the switching-on process and switching-off process of the power switching device may be inconsistent. Thus, in a series branch circuit for the power switching device, the power switching device with longer switching-on process and shorter switching-off process has to bear higher voltage, and the device with little leakage current has to bear higher voltage when being cut off. The higher voltage is detrimental to the work of the power switching device, and even the power switching device will suffer from breakdown damage when the voltage exceeds the withstand voltage limit value of the power switching device. Therefore, when the power switching devices are connected in series, special measures are required to be taken, and the voltage at two ends of the power switching device is limited in the safe range, for example, within the range of 70% of its withstand voltage limit value.
In the application of series connection of the power switching device, a currently used technical method is that a dynamic and static voltage-sharing absorption unit circuit is connected on each power switching device in parallel. As shown in FIG. 1, the dynamic and static voltage-sharing absorption unit circuit includes a diode, a capacitor, and a parallel voltage-stabilizing circuit, and can fulfill the voltage limiting function.
The prior technical method has the disadvantages that first, one power switching device corresponds to one parallel voltage-stabilizing circuit, more parallel voltage-stabilizing circuits are used, the number of elements is large, and the utilization ratio is low; second, each parallel voltage-stabilizing circuit is required to be debugged independently, and when the stabilized-voltage value is changed, all the parallel voltage-stabilizing circuits are required to be changed, thus the workload is heavy; third, the parallel voltage-stabilizing circuit adopts energy-consuming type design and needs to consume electric energy, thus the efficiency of the complete machine using the technology is reduced.